I don't really think that the end can be assessed as of itself, as
being the end, because what does the end feel like? It's like trying
to extrapolate the end of the universe. lf the universe is indeed
infinite, then what does that mean? How far is all the way and then if
it stops what's stopping it and what's behind what's stopping it? So
"What is the end?" is my question to you.
--David St. Hubbins, This Is Spinal Tap

APOCALYPSE

We have all read it, or think we have. It does not take long to read:
forty-five minutes, by my reckoning. The Bible's concluding chapter
begins directly, as a letter, quickly sashays into garish incoherence,
occasionally breaks into startling verse, and above all promises to
show "what must soon take place." It has been used most often, by
religious charismatics, as a kind of divine itinerary: Jesus catching
the 7:15 out of heaven, arriving in flames at Jerusalem's holy Mount
anon. Although America enjoys an early tradition of apocalyptic
thinkers (the saintly Roger Williams among them), a visiting Anglican
priest named John Nelson Darby was the first to smuggle into the New
Canaan an organized vision-known as Premillennial Dispensationalism -
centered around the book's theology of blessed annihilation. That was
in 1862. An American apocalypse, perhaps not entirely coincidentally,
was in the process of erupting. Despite chronic humiliations,
Premillennial Dispensationalists are with us still. A recent poll
disclosed that 59 percent of the American public believes that the
events projected by one John of Patmos will come true. Ancient
Christians apparently shared as strong a belief in the sturdiness of
John's prophecies. An annoyed Augustine of Hippo advised that "those
who make calculations" based on John's auguries should "relax your
fingers and give them a rest."

The vernacular name by which this queer book is often known,
Revelations, is wrong.
It is Revelation, or The Revelation to John (and note that
preposition). The book's Greek title is Apokalypsis, or Apocalypse,
which at its peaceable etymological nativity meant simply "unveiling."
Revelation comes to us from the Vulgate Bible, the translators of
which bowed Apokalypsis into Revelatio, the nearest Latin equivalent.
Drawing partly on the "small" apocalypse in the book of Daniel, partly
on Jesus' premillennial dispensationalist discourse on the Mount of
Olives, and partly on God knows what, Apocalypse has arguably
influenced the Christian worldview more than any other book in the

Bible-surprising for a gloomy phantasmagoria that the Good Book's
fourth-century compilers very nearly omitted. Yet here we are, still
helplessly attempting to clear away its two millennia of murk. The
same bewildering poll that unveiled a 59 percent rate of American
acceptance of Apocalypse also revealed that a quarter of Americans
believe that their Bible-despite a conspicuous textual absence of
airplanes, skyscrapers, or Muslims-predicted the horrific but hardly
apocalyptic attacks of two Septembers ago.

Lest we rest too easy, however, here is a numerical sequence that does
have bearing on the end of the world: 1950 DA. This is the designation
given to an asteroid that after fifteen more near-Earth passes may
eventually collide with our planet. If this rock fulfills its most
dreadful cosmic destiny, John's prediction of a sky vanishing like a
scroll rolling up into itself, his stars falling to Earth, his black
sun and earthquakes and lakes of fire and bloodred moon, will all come
quite horribly to pass, with one exception: there will be very few
Christians, very few people, left to exult in this much-delayed
fulfillment of prophecy.

BANG, WHIMPER, FIRE, ICE

Not excepting Robert Frost's famous eschatological either-or, the
world will end in fire and ice. Our sun, now enjoying the solar
equivalent of middle age, will, somewhere between two and five billion
years from now, run out of fuel and "go nova." It will first become
what is known as a red giant, a faltering star with a surface
temperature much lower than that which it currently has. This process
will unfortunately cause the sun to expand many times beyond its
present size, incinerating everything between it and Jupiter. The
sun's burning wave of helium will be traveling tens of thousands of
miles per second, and on Earth the sky will pass from blue to yellow
to the fire of atmospheric ignition in roughly the time it takes to
blink. Once this terminal solar spasm has run its course, the sun will
collapse into a small, cool, extremely dense star known as a white
dwarf. The remains of the inner solar system's flash-fried orbital
matter will then be encased within icy spherical coffins hundreds of
miles thick. Since very few species have ever managed to achieve a
life span anywhere near even one billion years, it is extremely
unlikely that recognizable inheritors of humanity's mantle will be
witness to this celestial endgame. "Going nova" is, for now, an
intellectual indulgence for astrophysicists and neurotic civilians
whose pessimistic affirmations are most comfortably expressed in
cosmic terms.

What, then, do we mean by The End? If we mean The End of the World
Itself, we need not concern ourselves. Setting aside the aftereffects
of our sun's demise fifty million centuries hence, Spaceship Earth is
stubbornly resilient. In geologic time, it is probably indestructible.
Do we mean The End of Life? If so, of what kind of life? Three and a
half billion years ago the primordially molten Earth cooled enough to
allow for the stabilization of its chief chemical components. The
result was swarms of prokaryotic cells that held planetary sway for
two billion years. One and a half billion years ago these simpletons
became eukaryotic cells, replete with fancy new cytoplasm and nuclei.
Another billion years passed before the occurrence of anything
resembling multicellular development. The story of life on Earth is
largely the story of stromatolites, organisms not likely to have
believed that the god of their understanding reserved a special place
for each of them in heaven. Do we mean by The End of the World, then,
The End of Multicellular Life? If so, the world has, by any practical
definition, already ended. Two hundred and fifty million years ago,
during the later Permian period, 95 percent of all Earth's life was
suddenly eradicated. Do we mean something as anthropocentric as The
End of Human Life? If so, how many of us have to die to signal the
collapse of the human endeavor? Perhaps we should ask the ghosts of
European Jewry. For the citizens of Hiroshima and Nagasaki, the world
certainly appeared to end on August 6 and August 9, 1945. For the
people of China the world ended with equally inarguable force in 1556,
when the worst earthquake in recorded history tore open, lifted up,
and smashed back into place the Shensi province, in the process
snuffing out 800,000 lives. More dramatically, 73,500 years ago what
is known as a volcanic super-eruption occurred in modern-day Sumatra.
The resultant volcanic winter blocked photosynthesis for years and
very nearly wiped out the human race. DNA studies have suggested that
the sum total of human characteristics can be traced back to a few
thousand survivors of this catastrophe.

Earth itself is the most ambitious mass murderer in the galaxy. Its
pleasant, cloudy face should be slapped onto a WANTED poster, and
soon: this lunatic is out to get us. As the volcanologist Bill McGuire
notes in A Guide to the End of the World, an elegant volume of
nervous-breakdown-inducing persuasiveness, "The Earth has been around
just about long enough to ensure that anything nature can conjure up
it already has." As recently as 1902, Mount Pelee in Martinique
exploded, vaporizing the town of St. Pierre with a glowing avalanche
of skin-melting gas and lava flowing as fast as a cheetah at full
sprint. Thirty thousand people died; the blast's two survivors were
likely to have regarded any notion of the world's end with seared,
unblinking eyes. Earth is cratered with somewhere between 1,500 and
3,000 volcanoes, and at least one explodes each day. Most of North
America's volcanoes have been dormant for centuries--one particularly
angry specimen detonated 450 million years ago and evaporated an area
the size of Egypt-and this is certainly good news. The bad news is
that the volcanoes that have gone speechless for the longest periods
of time tend to have the most to say when they finally break their
silence. The long-dormant Indonesian volcano Tambora blew off its peak
in 1815, with an explosion heard 900 miles away. Twelve thousand
Indonesians died as a direct consequence of the eruption, and another
80,000 perished due to famine. The atmosphere was crowded with so much
ash and sulfur that 1816 became known as the "year without a summer."
England saw its heaths frosted through July, allowing Mary Shelley the
proper frame of mind to complete the wintry Frankenstein.

That such natural disasters seem, in the Western mind, exclusive to
primitive people in far-off lands is mostly a fluke of our current
distributional good fortune. This quietly unapologetic stance, which
braids laissez-faire eugenics with Western exceptionalism, is in fact
a highly callous form of disaster denial, for yesterday's
village-erasing lava flow in Sumatra is tomorrow's super-eruption in
British Columbia. Human civilization as a whole has grown amid a long
breather from the kinds of natural calamities that, 10,000 years ago,
left our freshly de-Ice Aged planet broiling beneath junglish
centigrade from Ellesmere Island to Admundsen-Scott Station. It would
probably require a psychiatrist magically privy to the workings of the
mass mind to explain why we as a species seem resistant to believing
that such disasters could recur, especially when many feel so
ominously near. The planetary conditions we are today seeing unfold
-alarming rises in human population, sea level, and temperature (in
100 years Earth will be hotter than it has been at any time in the
last 1,500 centuries) - could very well be the overture to nature's
awful new symphony.

Yes, the old doomsday warhorses of overpopulation and global warming
(which actually should be called anthropogenic warming) are still
bedeviling us. Little more can be said of global warming, other than
to note that it is well under way and that those who maintain
otherwise are the meteorological equivalent of creationists. In the
case of overpopulation, no one can deny that the world's inhabitants
doubled between 1960 and 2000. Whether this is merely bad or
catastrophically bad is still unclear. Equally unclear is whether
overpopulation (a term too often used as code for too many brown and
yellow people) will result in a long-feared drain of world resources.
But the truth remains that this sudden explosion of population is,
most basically, unprecedented in human history.

One runs the risk of sounding like an idiot, or worse, when pondering
the sandwich-board finalities of The End. Consider the most famous
Cassandra of the 197Os, Hal Lindsey, author of The Late Great Planet
Earth, the best-selling book of that deeply regrettable decade. (It
still, somehow, sells around 10,000 copies a year.) His predictions of
the rise of a single world religion, a Soviet-Ethiopian invasion of
Israel, and the obliteration of Tokyo, London, and New York, all
systematically unfulfilled, have made him a laughingstock along the
lines of pet rocks. It is with a sense of near torment, then, that I
report Lindsey's Reagan-era sinecure as a consultant on Middle Eastern
affairs. Our current decade's disseminators of global doom, Jerry B.
Jenkins and Tim LaHaye, authors of the projected twelve-novel cycle
Left Behind, are hardly improvements. The series' ninth
installment, the accurately titled Desecration, was the
best-selling novel of 2001. These jaw-droppingly substandard books
peddle a religion of fear and despair (passenger-laden airliners,
whose faithly pilots are Raptured up to heaven, are allowed to crash),
partake of some truly shocking Jew-baiting (Nicolae Carpathia, the
books' antichrist, lately of the United Nations, is helped along by a
sinister cabal of "bankers"), and pile up body counts with
slasher-flick glibness (the nuclear immolation of Chicago merits a
single paragraph). And just as Hal Lindsey filled the mental shoals of
Ronald Reagan with his insights into the Middle East ("If you stumble
over some of these unpronounceable ancient names, glance at your daily
newspaper and notice the tongue , twisters assigned to people today"),
Tim LaHaye served as the co-chairman of Jack Kemp's 1988 presidential
campaign.

Apocalypticism, it hardly need be said, drags out of humanity all that
is small and terrible and mean. It also drags out what is worst about
God, for whom love seems an infrequent mood. But the apocalyptic
vision speaks to us, and has spoken to us, for thousands of years. It
has allowed the quatrain-scribbling Nostradamus to live indefinitely
off the obscurantist residuals of human anxiety. It has allowed some
in the Muslim world to regard figures as disparate as Pope Innocent
III and George W. Bush as the long-feared Dajjal. It has allowed
George W. Bush, arguably the worst president in American history, a
surreally nonexistent pretext for world war. It seems we are all going
a little nova. In July of 2002, 13,500 men and women of the army,
navy, air force, and marines engaged in a war game called Millennium
Challenge. That exercise, the largest of its type ever conducted, was
based on a rogue military commander staging a coup in an
earthquake-riven Middle Eastern nation. Make that
millennium-challenged: the rogue military commander won, until the war
game's rules were reset.

Is a war game rigged to ensure victory a reasonable depiction of The
End? Or does The End consist merely in extrapolating the size of the
volcano needed to turn Jakarta into a cinder or calculating how many
feet of water New England will be sitting under once the polar ice
caps melt? Is it discerning the arctic climate of Great Britain once
the Gulf Stream vanishes or projecting the megadeath of a nuclear
exchange between Islamabad and Delhi? It will be no easy thing to
eradicate six billion human beings, the most wide-ranging, adaptable,
and notoriously intelligent creatures to which the planet has ever
played host. But whereas human beings are tough, the civilizations
they form are appallingly delicate. Here is the buried psychological
certainty - I may die, but human culture will endure - that the
apocalyptic vision upends.

Thus we have entertained, and still entertain, The End
at the hand of God. We have been plagued, and will be plagued, by The
End at the hand of the planet itself. What seems most likely is that
The End of the World will mean the reduction of humanity to a
dangerously puny number, whereupon our planet will become another
planet altogether, something we will live upon but that will no longer
belong to us. Perhaps that future will be simpler and more peaceful,
perhaps toothier and rougher. Star Trek or Mad Max? If
we do proceed apace and engage in a global swap of ICBMs, who among us
could argue that we did not have it coming? But imagine the different,
far less personal end embodied by objects such as 1950 DA. Who among
us would argue that we had that coming?

This new End means the loss of God, and not only because it creates a
kind of philosophical yard sale in which everything must go. No, God
will be discarded long before The End truly begins. We have already
rummaged off not a few versions of Him. The hirsute old
thunderbolt-hurlers to whom we long paid tribute - priapic Zeus, testy
Yahweh-cannot map the genome, or tell us our past, or even explain our
future. Only we can do that. "Playing God" previously meant the
ability to take life, a feat we too can now achieve with spectacularly
divine wrath. The old God, whatever his alias, is dead, and the God we
are left with today is of little discernible help. Or perhaps God is
nothing more than a mile-wide chunk of cosmos-wandering silicate,
serenely floating right toward us. Was it all a mistake?

I HEARD THE LEARN'D ASTRONOMER

NASA's Deep Space Network antennae at the Goldstone Deep Space
Communications Complex are spread out along twenty-six miles on the
grounds of Fort Irwin, a United States Army training center in the
desert wastes between Los Angeles and Las Vegas. I am traveling to the
complex, which is fenced off from the surrounding military base, with
Steve Ostro, a radar astronomer who works out of NASA's Jet Propulsion
Laboratory (JPL), in Pasadena, California. Ostro is a handsome,
southern-California-fit man in his early fifties. His resemblance to a
more dashing version of Russell Johnson's Professor from Gilligan's
Island is spoiled very slightly by his glasses, which although not
unflattering are as thick as bulletproof glass. As we chat we pass by
small road signs that say things such as TANK XING,
billboards that read THE ABCS OF SAFETY:
AlWAYS be CAREFUL, and hard, dried-up lakebeds that serve as
landing strips. Often, Ostro tells me, he has seen military exercises,
some of them live-fire, playing along these bleak horizons: Black Hawk
helicopters cruising 100 feet off the ground at 130 miles per hour,
tank columns advancing upon coyotes and gophers while huge volleys of
artillery boom across the vacant desert sky.

Goldstone's centerpiece antenna, the DSS-14, was built in 1966 to
receive communications from, and plot the movements of, early NASA
missions to Mars. The monolithic dish dominates the surrounding,
studies-in-brown terrain due to both its twenty-four-story height and
(despite a crow infestation) its gleaming polar whiteness. When
pointed straight up, as it is now, it strongly resembles a chandelier.
The nine-million-pound antenna rests atop a round pedestal and has a
pointing precision measured in something called millidegrees. Small,
anonymous buildings wreathe the antenna, some housing dozens of tall
metal lockers that contain telemetry equipment used to communicate
with deep-space vehicles, others crammed with chugging generators that
provide the antenna enough wattage to power a small town. Inside these
buildings it is kept very cold-a little less than fifty degrees-to
prevent the computers from overheating. Not surprisingly, a low and
vaguely doomed hum pervades Gold-stone's every nook and turn, rather
like what one hears aboard a cruising 747, only much louder, and many
who work here complain of headaches. After two days I will be
complaining of one myself.

For the last several years Ostro has been coming to Goldstone to map
by radar our solar system's larger known asteroids. The DSS-14 sends
out a beam far too thin to discover new asteroids. That duty is
handled by the Near-Earth Asteroid Tracking project at JPL, which
reports its findings to the Minor Planet Center, in Cambridge,
Massachusetts. If the observed object is a new asteroid - occasionally
older asteroids go missing and are mistaken on reappearance for new
ones - MPC gives the object a number based on the year and month and
order in which it was discovered. (1950 DA, for instance, was
discovered in 1950 during the second half - each half-month is given a
letter - of February. The A indicates that it was the first asteroid
found in that half-month.) NEAT observes the new asteroid until its
orbit can be reasonably determined, at which point Ostro takes over.
Powered by a half-million watts, the antenna sends out into space a
beam with the angular resolution of the human eye. The beam hits the
targeted asteroid uniformly, scattering in every direction; only a
small portion of the beam's energy ever makes it back to Goldstone.
But this tiny fraction provides Ostro and his colleagues with enough
information to determine the asteroid's velocity, size, and likely
structural components. Often enough, they are able to use the
collected data to create a grainy but nevertheless beautifully
revealing image of the object.

As we walk into the lab housed in the antenna's pedestal to meet with
Ostro's colleagues, he tells me he finds radar astronomy "an
unbeatable experience." What before had been only an infinitesimal
point of light attached to an ungainly number suddenly becomes a tiny,
detailed world. I begin to understand his remark, made way back on
Interstate 15, that he stopped reading the sci-fi novels he loved as a
teenager when the science he was involved in became more interesting
to him than fantasy.

Inside the pedestal I meet Lance Benner, Jon Giorgini, and Ray
Jurgens, JPL scientists highly adept in the different areas of
astronomy and planetary science that allow the team to apply an
astonishing interpretive breadth to the antenna's radar readings.
Jurgens, the oldest of the men, seems to be providing a good deal of
support merely by standing on the lab's fringes and quietly observing.
All of them are distracted, as the antenna's transmit-receive cycle is
about to begin. I stand to the side, staring at a few taped-up
computer-generated images of Goldstone-mapped asteroids.

Very little is known about asteroids. The spin states, shapes,
geological compositions, surface characteristics, and collisional
histories are, for the vast majority of identified objects, still a
mystery. Only with radar imaging is much of this data, accomplished
one object at a time, finally becoming clear. The asteroids' irregular
shapes give each something resembling a personality: Kleopatra is
shaped like a dog bone, Eros like a rutabaga, Geographos like a turd.
Some rotate evenly, others like badly thrown footballs.

The lab's equipment appears, to me, strangely antiquated. Somehow our
technology improves but gets no closer to the touch-screen sleekness
of cinematic futurism. Little red and white lights blink on the
hulking computers' faces alongside small screens active with greenish
waveforms. A spray of cables hangs from seemingly every panel.
Jurgens, who has worked at Goldstone since the 1970s walks over and
explains that much of this equipment is twenty years old. Later
inquiries as to how adequately NASA funds Goldstone's radar astronomy
work-it is about one ten-thousandth of NASA's overall budget-will
result in meaningful silences.

Ostro escorts me to another computer at the room's far end, the
real-time sawtooth display of which will soon show us the
electromagnetic Doppler frequency the antenna is receiving back from
the asteroid. This information will be used, Ostro explains, to
calculate the asteroid's orbit uncertainty. "We go through this
process where we have a projected orbit. We see how good the
prediction was, and then we make a better orbit and a new prediction
of uncertainty. There's always uncertainty, and that's one of the
really interesting domains of this whole problem. What is the
uncertainty, and how do we reduce it? That's why we're here."

I am here, I remind him, to find out about the chances of one of these
asteroids colliding with Earth. But this is where the uncertainty
comes in, he tells me. Every asteroid travels along a path we can
determine using orbital trajectories, but within that trajectory there
exists an error ellipse in which we cannot be sure where the asteroid
will be. This ellipse can be many hundreds of thousands of miles in
width, which makes reducing the uncertainty of an object that passes
near Earth that much more important. Asteroid 1997 XFll, the asteroid
we will be observing today, has a curious history of uncertainty. Five
years ago, 1997 XFll was predicted to have a small but nonzero chance
of hitting Earth in 2028. Whether this was due to hasty "back of the
envelope" calculations or a real uncertainty in its error ellipse is
still debated. What is known is that the media frenzy was so immediate
("Killer Asteroids!") that NASA created the Near-Earth Objects Office
to handle future impact threats. It was later determined that 1997
XFll had no chance of hitting Earth, and before the day ends we will
know everything else there is to know about this defanged rock. "

A lot of the confusion about this topic," Ostro goes on, "ultimately
comes down to miscommunication. All of this is unfamiliar and
intrinsically arcane and inaccessible and beyond the experience of
humanity." (A sample from a paper Ostro gave me: "This is simply
saying that a survey system with a limiting magnitude mlim
= 20 will achieve the same completeness of absolute magnitude H = 20
objects as a system with mlim = 19 will achieve of H = 19
objects.") I ask Ostro if he personally worries about the day they
discover an asteroid that has a high probability of hitting Earth. He
is silent for such a long time that I ask again. "Let me rephrase your
question," he says. "Is there a God?" After some uncomfortable
laughter on my part, Ostro tells me about 1950 DA, the only large
asteroid currently known that has a nonzero chance of colliding with
Earth before the next millennium. If it does strike, it will impact
the North Atlantic just off the U.S. coast in March of 2880. "It's a
little bit of a stretch to say it might hit the Earth-the probability
is 1 in 300-but it's the most dangerous object we know. Now, do we
care about that? Should anybody care at all about the fact that an
asteroid might hit Earth nearly a millennium from now?"

I imagine standing in a room with 300 people, then being told that one
of us will be taken outside and shot. I tell Ostro that I think I can
care about that.

"What if I had said, 'Well, we found an object that has a pretty good
chance of hitting the Earth in 500,000 years.' Would that concern you?
Should we care about that?"

I admit that I have a hard time gathering the emotional momentum that
allows my concern to travel ahead a half million years.

Ostro nods. "What it comes down to is that this is a very new kind of
topic, and it's hard to get one's bearings thinking about it, much
less for society to decide whether to worry and spend money on it. And
if so, how much, and how?'

The lab's small, encaged red light begins to flash in alert: the
antenna is finally transmitting its half-million-watt, pencil-thin
beam of energy seven million miles into deep space. Its round-trip
time back to Goldstone will take a little under eighty seconds. As we
wait, I find myself thinking of Whitman's "When I Heard the Learn'd
Astronomer." In the poem Walt grows so "tired and sick" of an
astronomer's "charts and diagrams" that he goes out "In the mystical
moist night-air, and from time to time,/Look'd up in perfect silence
at the stars." I wonder what poem he might have written had he known
that some of those stars had the potential to end poetry and
everything else, for all time.

How did we get here? In 1178. a monk in everything else, for all time.
Canterbury, England, recorded the testimony of two men who witnessed a
"flaming torch" spring up off the face of the moon, which "writhed, as
it were, in anxiety," then "took on a blackish appearance." What these
men saw, some scientists believe (the issue is debated), was the
formation by an asteroid collision of the moon's youngest known
crater, Giordano Bruno, named in honor of a defrocked Italian
philosopher-priest. The explosion had the estimated force of 120,000
megatons, equal to 120 billion tons of TNT. Hiroshima was a mere 15
kilotons. The greatest man-made explosion in history, a Soviet nuclear
test on the Arctic island Novaya Zemlya in 1962, was 60 megatons. If
every nuclear device on the planet were somehow to explode at the same
moment, no more than 20,000 megatons would be unleashed. The formation
of Giordano Bruno, if that is indeed what the two witnesses saw,
marked perhaps the first time in recorded history that human beings
observed what is now known as a large-body impact. The next would
occur more than 800 years later, when two dozen fragments of a
shattered comet would explode on the surface of Jupiter. Not even the
intervening centuries of scientific advancement would allow us any
true comprehension of the destructive potential of large-body impacts.
Faced with the effects of 20 megatons of explosive energy for every
man, woman, and child on Earth, the mind is quickly beaten into
something misshapen and medieval.

The term "asteroid" means "like stars," stars being what earlier
humankind most often mistook asteroids for. When an asteroid breaches
Earth's atmosphere it becomes a meteor; when it strikes Earth's
surface it is called a meteorite. A "shooting star" is typically
envisioned as a midsized burning chunk of speeding rock, when in fact
most shooting stars are no bigger than a grain of sand. The speed at
which they travel, and the opposing force of Earth's atmosphere, cause
the particles to explode. The streaks of light we see in the night sky
are these particles' violently released energy.

The solar system's primary asteroid repository whirls in a formation
known as the asteroid belt, which lies between Mars and Jupiter, the
latter possessing our system's second most powerful gravitational
force, after the sun. Most asteroids are the fragmentary remains of
the same cosmic deus ex nihilo that discharged rock and matter
across the galaxy several billion years ago. In our solar system
alone, as many as a trillion pieces of debris once floated around the
perpetually contained hydrogen explosion we know as the sun, most of
which were bashed into oblivion by collisions. Only nine of these
space rocks were large enough to form atmospheres, and only one is
known to have developed complex forms of life. This did not make them
invulnerable. A Mars-sized object slammed into the nascent Earth
billions of years ago, for instance, and the drama of its effect can
be appreciated by the fact that it threw off a huge, wounded, molten
glob that froze, was captured by Earth's gravity, and eventually
became the moon. Because of this ancient demolition derby, the solar
system is presently a much more open place, and collisions are far
less frequent.

The first identified asteroid, Ceres, was found in 1801. By 1900
astronomers had located 462 more. All but one (Eros, discovered in
1898) were asteroid-belt objects. In the last 100 years, 150,000 more
have been pinpointed and the orbits of 52,000 accurately determined.
Most of this orbital surveying was accomplished after 1968, when the
asteroid Icarus passed within four million miles of Earth and first
caused astronomers to ponder the possibility, if not the likelihood,
of large-body impacts.

Asteroids are categorized into several classes. S-types, which
dominate the inner half of the asteroid belt, are composed of stone
and silicate materials. C-type asteroids, which take up most of the
outer half, are dark rocks rich with complex organic compounds called
carbonaceous chondrites. P- and D-type asteroids are the farthest away
and, consequently, the most composition-ally mysterious. The belt's
nearest asteroids are M-types, highly reflective iron-nickel objects
that scientists believe are tantalizing atavists of planetary cores.
In Ostro's office at the Jet Propulsion Laboratory he handed me a
black-and-gold chunk of a billion-year-old M-type asteroid. Although
it was only a little larger than a compact disc, my hand nearly hit
the floor. Its density, Ostro explained, was twice that of any object
of terrestrial origin, and to cut it open would reveal an interior as
bright as freshly forged steel. The smallest known M-type asteroid,
3554 Amun, with a radius of only 500 meters, is thought to contain $1
trillion worth of nickel, $800 billion worth of iron, and $700 billion
worth of platinum. Since towing it back to Earth would be
counterproductively expensive, and likely to result in the collapse of
the world market for fine metals, these figures are, for now, lit with
little more than the neon of sci-fi dreams.

In its journey around the sun Earth passes through the orbits of
twenty million asteroids. Many of these Earth-crossers are called
near-Earth asteroids. NEAs much smaller than 100 meters wide are
basically undetectable but for a fluke of stargazing luck;
unfortunately, an object of only, say, 90 meters possesses the
collisional capability of roughly 30 megatons of explosive energy, a
figure that is dreadful but globally manageable. NEAs larger than 100
meters are thought to number 100,000, a fraction of which have been
located; in the event of an impact these could effect serious global
climate change. Around 20,000 NEAs are large enough, individually, to
annul a country the size of the Czech Republic. The number of NEAs
bigger than one kilometer in diameter is currently thought to be
around 1,000. At astronomers' current rate of detection-roughly one a
day-a survey of the entire population of one-kilometer NEAs will be
complete within the next decade.

This one-kilometer threshold is important, for asteroids above it are
known as "civilization- enders." They would do so first by the kinetic
energy of their impact, striking with a velocity hitherto unknown in
human history. The typical civilization-ender would be traveling
roughly 20 kilometers a second, or 45,000 miles per hour - for
visualization's sake, this is more than fifty times faster than your
average bullet - producing an impact fireball several miles wide that,
very briefly, would be as hot as the surface of the sun. If the
asteroid hit land, a haze of dust and asteroidal sulfates would
enshroud the entire stratosphere. This, combined with the soot from
the worldwide forest fire the impact's thermal radiation would more or
less instantaneously trigger, would plunge Earth into a cosmic winter
lasting anywhere from three months to six years. Global agriculture
would be terminated, and horrific greenhousing of the climate and mass
starvation would quickly ensue, to say nothing of the likely event of
world war - over the best caves, say. In the event of a 10-kilometer
impact, every- thing within the ocean's photic zone, including
food-chain-vital phytoplankton, would die, but this would hardly
matter, as the deadly atmospheric production of nitrogen oxides, which
would fall as acid rain, would for the next decade poison every viable
body of water on Earth. Chances are, however, that the impact would be
a water strike, as 72 percent of meteorite landings are thought to
have been. This scenario is little better. A one-kilometer impact
would, in seconds, evaporate as many as 700 cubic kilometers of water,
shooting a tower of steam several miles high and thousands of degrees
hot into the atmosphere, once again blotting out incoming solar
radiation and triggering cosmic winter. The meteorite itself would
most likely plunge straight to the ocean floor, opening up a crater
five kilometers deep, its blast wave cracking open Earth's crust to
uncertain seismic effect. The resultant tsunami, radiating outward in
every direction from the point of impact, would begin as a wall of
water as high as the ocean is deep. If a coastal dweller were to look
up and see this wave coming he or she would be killed seconds later,
as it would be traveling as fast as a 747. Of course, these are all
projections based in physics, and can be scaled either slightly up or
slightly down in their potential for global destruction. As the
paleontologist David M. Raup puts it, "The bottom line is that
collision with a. . . one-kilometer body would be most unpleasant."

Although one-kilometer impacts (at least several thousand megatons)
are thought to occur once every 800,000 years, with 200-meter objects
(1,000 megatons) striking once every 100,000 years and 40-meter
objects (10 megatons) striking once every 1,000 years, only a handful
of professional and amateur astronomers are currently watching the
skies. Nearly half of the asteroids believed capable of destroying one
quarter of humanity remain uninventoried. Not until 1998 did the U.S.
Congress direct NASA to identify, by 2008, 90 percent of all asteroids
and comets greater than one kilometer in diameter with orbits
approaching Earth. Unfortunately, the government agency - of any
government, anywhere - that would react to and be expected to deal
with the likelihood of an asteroid impact does not currently exist.
The impact threat is what Ostro calls "low probability and high
consequence," and bureaucracies scatter like roaches from the
kitchen-bright possibility of severe consequences. We need only to
consider the disgraceful games of administrative duck-duck-goose
played in the aftermath of comparatively smaller disasters, such as
the terrorist attacks of September 2001, to recognize the federal
unwillingness to counter its own congenital laxity.

Nonetheless, as I wait with Ostro, Benner, Giorgini, and Jurgens for
1997 XFll's first measurements to appear on-screen, I experience
something like patriotism. The United States is currently the only
nation in the world doing anything about the possibility of asteroid
impacts. I am standing with a group of interstellar Paul Reveres. When
I mention this to Ostro he shrugs. "The world owes a great deal more
to the United States than is commonly supposed," he says, staring at
the screen. "And the United States should be very proud of itself for
supporting the research it does, and being the first to take this
seriously."

The bandwidth begins to come alive, and after a little while Ostro is
excitedly pointing things out. "It looks like it's a fast rotator; it
looks like it's more or less spheroidal; it's not an elongated object.
We guessed its size at a little more than a kilometer, and that looks
to be about right." While Benner begins the slow process of creating a
pixelated image of 1997 XFll, Giorgini sits down at a computer console
to exploit the new radar information to reduce the uncertainties of
the asteroid's orbit by a factor of 5,000, using its current position
to integrate its orbit backward and forward in time. Jurgens laughs
and says that twenty years ago this would have required two years of
computation. When I ask if that resulted in more or less error,
Jurgens says, dryly, "There was a lot of error."

Giorgini's computer is essentially putting to use every practical
thing that human beings have learned about mathematics and physics
over the last 1,000 years. While we wait for it to provide a table of
the asteroid's journey through time and space, I ask Giorgini if he
ever worries about impacts. "It's unlike almost all other natural
disasters," he says. "We can't do much about a hurricane, and we can't
do much about volcanoes, but there's a predictability to asteroid
impacts that will give us an interval, and the interval is comparable
to a human lifetime. Some guy working alone in his basement could
design some killer bacteria without anybody knowing about it. Whether
an asteroid hits us before then, I don't know. You can't worry about
everything." When I ask about the day 1950 DA's nonzero likelihood of
impact came up on Goldstone's screens, he gives his head one brisk
shake. "That was very exciting."

Suddenly Ostro tells me that if 1997 XFll's impact hazard unexpectedly
comes up nonzero I will be escorted into the desert and left for the
coyotes. I ask whether impact "cover-ups" fall under the heading of
right-wing or left-wing conspiracy. "Both wings flap together," Ostro
says.

The information we have been waiting for begins to unscroll in several
columns down Giorgini's screen. "So here we have the close-approach
table," he says. I determine, privately, to call it the Holy Shit
Table. "Notice these are all zeroes from the year 1900 up through the
year 2100. And here in 2028"-its former impact year-"you can see it's
about two and half lunar distances from Earth." He executes a few
quick keystrokes and brings up another table showing the asteroid's
close approaches, or "planetary encounters," every year from 1627, a
year before Salem was founded on Massachusetts Bay, to 2228. Nothing
but zeroes in the Holy Shit Table, not only for Earth but for the
moon, Venus, and Mars as well. Until then, at least, we are safe from
1997 XFll. This, Ostro says happily, is as close to time travel as we
are likely to get. Thanks to one radar reading we have been awarded
the virtual travel diary and itinerary of an object seven million
miles away. We are safe.

THE GIGGLE FACTOR

Impact theories are not new. One of the first scientists to argue in
favor of them was Dr. Grove Karl Gilbert, in 1893, though he went only
as far as placing past asteroid impacts on the moon. The resistance to
dealing with the implications of Earth-based impacts, however, is
almost as old as science itself. The planetary scientist John S.
Lewis, author of Rain of Iron and Ice (somewhat plaintively subtitled
The Very Real Threat of Comet and Asteroid Bombardment), credits this
resistance to the "giggle factor," a "half-suppressed hysteria that
arises from an emotional inability to deal with the truth." Whereas
Isaac Newton was obsessed with the end of the world, dusting
relentlessly for the sulfuric cultural fingerprint of the antichrist,
he dismissed the (even then) growing evidence for large-body impacts,
believing that God had put everything in its proper celestial place.
When a meteorite struck Weston, Connecticut, in 1807, two Yale
professors verified the strike as extraterrestrial. Thomas Jefferson,
after parsing their report, allegedly said that he "would find it
easier to believe that two Yankee professors would lie, than that
stones should fall from the sky."

Earth is home to more than 170 known impact craters, some as old as
two billion years. Erosion has undoubtedly erased dozens more. For
many veers these craters, including the startlingly well Preserved
Meteor Crater in Arizona, were said to have resulted from
"crypto-volcanic" (hidden volcanic) activity, though some argued that
this was impossible. On account of the controversy surrounding Meteor
Crater's origins, maps made of Arizona prior to its statehood in 1912
omitted it altogether, no small feat of obfuscation for a formation
nearly one mile in diameter. Since the few impact studies being
conducted at that time saw researchers dropping marbles into bowls of
oatmeal and recording the ratio of displaced mush vis-Ã -vis the size
of the offending cat's-eye, it is easy to understand why the crater's
enormity and almost perfect roundness proved so baffling.

Since 18 12, when a twelve-year-old girl named Mary Anning found a
seventeen-foot-long Ichthyosaurus fossil along the cliffs of Dorset,
humankind has been forced to come to terms with the upsetting evidence
that many hundreds of thousands of startling and, sometimes,
frightening genera came before it. The secondary recognition-that
something caused these creatures to go extinct on a massive scale-soon
followed. Mass extinctions are clearly evident in the fossil record
and were duly noticed by nineteenth-century geologists (or
"undergroundologists," as they were then known). They made these
extinctions the basis of division (Cambrian, Devonian) within the
geologic time scale. The question about mass extinctions was one of
agency. Georges Cuvier, a brilliant French paleontologist and until
Charles Darwin the most famous scientist of the nineteenth century,
settled upon a "Doctrine of Catastrophes," which envisioned violent
"revolutions" that all but swept clean ancient worlds of life. The
events were "so stupendous," Cuvier wrote in 1821, that "the thread of
Nature's operations was broken by them and her progress altered."
Delighted Christians took this as proof of the Noachian deluge.

The opposing view of the 'world's prehistory was represented by the
equally brilliant Scottish geologist Charles Lyell, who began as an
admirer of Cuvier. Lye11 believed that "we are not authorized in the
infancy of our science, to recur to extraordinary agents." Lyell's
view of the fate of species, most forcefully explained in his
Principles of Geology (1830), was derived from a theory of slowly
accumulating processes. Extinctions, then, were gradual affairs, and
not subject to fantastical caprice. Lyell's view became known as
uniformitarianism and proved so convincing that, within decades, to
espouse any version of Cuvier's catastrophism became the scientific
equivalent of wearing a tinfoil hat and claiming that streetlamps were
issuing death rays. The gradualist interpretation of the world - which
has, in most disciplines, served science well - came to dominate the
study of mass extinction. In this century the demise of the dinosaurs
has been associated with causes ranging from small ratlike mammals
eating Tyrannosaurus eggs to gamma-ray bombardment from an exploding
supernova to dinosaurs becoming too big to mount their partners to
gradual climate change. (I remember vividly, as a young boy, weeping
over an educational cartoon filmstrip that showed Diplodocus
staggering through a desert and collapsing.)

But on June 30, 1908, something happened that would slowly begin to
change the terms of this debate. Over the skies of a Siberian riverine
area known as Tunguska (a word that has "9/l l"- like resonance among
astronomers), a small stony meteor no wider than sixty yards punched
through the upper atmosphere and, due to aerodynamic pressure,
detonated four and a half miles above the surface. The explosive force
was that of 800 Hiroshimas and shook the Earth with the ferocity of a
magnitude-eight earthquake. Seven hundred square miles of Russian
woodlands were incinerated in seconds. That evening Europe's sky was
so bright that there were reports of games of midnight cricket being
played by uneasy Londoners. Amazingly, only two people were reported
to have died in the Tunguska blast, as the area was mostly
uninhabited. Had the asteroid been delayed by four hours, the
explosion would have occurred over St. Petersburg (and, perhaps,
prevented Soviet Communism). Russia was under considerable upheaval at
the time - Czar Nicholas II had dissolved two successive parliaments
in the preceding two years - and no one journeyed to Tunguska to
investigate the event before 1927. Not until years after Hiroshima,
Nagasaki, and the repeated irradiation of New Mexico and Kazakhstan
were the physics of massive explosions properly understood, and the
strangely craterless Tunguska site was subsequently adduced to have
been caused by an asteroidal airburst, an event more common than one
might suspect. The U.S. Defense Support Program's comprehensive
satellite system, ostensibly used to provide for the global tracking
of enemy bombers and missile launches, detects a dozen ten- to
twenty-kiloton explosions in the upper atmosphere every year. A 1963
blast of 500 kilotons above Antarctica was initially mistaken for a
nuclear test by South Africa. Until the popular emergence of UFO
sightings, these explosions were commonly observed and reported by
civilians.

Nonetheless, the danger posed by large-body impacts was discounted
well into the 1950s a time when a theory as elementary as continental
drift was seeing its first acceptance. Nuclear weapons were still in
their nativity, after all, and humankind had several thousand silos'
worth of exterminating angels to worry over. By 1964 it was commonly
believed that Earth possessed only six verified impact craters, and
the impact-mass extinction link was ridiculed. In 1970 the Canadian
paleontologist Digby McLaren went public with his belief that the
possible culprit of the mass extinction at the end of the Devonian
period, 365 million years ago, the fourth most intense mass extinction
of all time, was impact-related. The Nobel laureate Harold Urey made
another claim for impacts and mass extinctions in the journal Nature,
in 1973, but since Urey was a chemist his research was thought
suspect. Then, in 1980, Luis Alvarez, Walter Alvarez, Frank Asaro, and
Helen Michel published in the journal Science an article entitled
"Extraterrestrial Cause for the Cretaceous-Tertiary Extinction."

The Cretaceous-Tertiary extinction, which occurs at what is known as
the K-T boundary (C having already been secured by the Carboniferous
period), marks the sixty-five-million-year-old point at which the
dinosaurs go AWOL from the fossil record. Even at their acme of
diversity (small children should probably stop reading now) no more
than fifty species of dinosaurs, a decidedly trivial portion of
Mesozoic Era life, were alive at one time. At the K-T boundary, as few
as twenty-five saurian species were left. None survived past it. Of
the mammalian species, perhaps ten or fifteen survived the K-T mass
extinction, commonly thought to be the second most profound of its
kind. Similar losses are mirrored in the fossil record of every
species alive at the time.

The Alvarez group determined that the K-T boundary clay had an
anomalously high incidence of the element iridium. Since iridium is
roughly 5,000 times more abundant in extraterrestrial objects than in
Earth's accessible crust, it seemed clear that something
cataclysmically extraterrestrial in origin occurred at the K-T
boundary, an event that showered Earth with the element. These
findings were attacked for several years, leading the New York Times,
in 1985, to issue a now famously dyspeptic editorial. "Astronomers,"
the Times scolded, "should leave to astrologers the task of seeking
the cause of earthly events in the stars." (Stephen Jay Gould
brilliantly ridiculed the Times by writing a fictitious editorial
dated 1663: "Now that Signor Galileo . . . has renounced his heretical
belief in the earth's motion, perhaps students of physics will.. .
leave the solution of cosmological problems to those learned in the
infallible sacred texts.") Not until the discovery, several years
later, that the "shock metamorphism" of large-body impacts (and
nuclear explosions) can form two separate minerals, stishovite and
coesite, and that many suspected impact sites had high concentrations
of both, did the hypothesis begin to win converts. A mid-1980s poll
revealed that 90 percent of American scientists quizzed accepted that
impacts occurred, though only 4 percent accepted an impact as the
explanation for the K-T mass extinction. Previous beliefs that
large-body impacts affected only the "lethal area" in the direct
vicinity of the strike were, however gradually, abandoned. Impacts may
happen, it was at last conceded, but they were freakish, remote events
that did not script the fate of biology.

In 1990 the eminent late astronomer Eugene Shoemaker presented a paper
to the Geological Society of America that demonstrated, as never
before, the sheer number of Earth-crossing asteroids in our solar
system, concluding that "Earth resides in an asteroid swarm." With
these six words, Shoemaker finally, viscerally stated what scientists
had been willing to accept only with decorous academic distance:
millions of pieces of rock, some of them massive, were flying past
Earth at staggering speeds, and sometimes these rocks hit us. All we
had to do was have a look around at our beat-up planet for the
well-documented proof. Opponents of the impact hypothesis suffered
further attrition when, under the tip of the Yucatan Peninsula, near
the Mexican port city of Progreso, a sixty-five-million-year-old
crater 120 miles in diameter was linked decisively to the K-T mass
extinction. The crater, called Chicxulub, had been found as long ago
as 1978, but because its discoverer, an oil-industry geologist named
Glen Penfield, was not an academic, converts were hard-won. By the
early 1990s Chicxulub had become widely accepted as the scar of a
large-body impact that finished off the dinosaurs and 75 percent of
all other life on Earth.

Currently almost 1,000 potentially civilization-ending NEAs are known
to have orbits passing within five million miles of Earth. It is
statistically likely that most of us will live to see another impact,
however small or large. In 1996, asteroid 1996 JA1 came within 200,000
miles of hitting Earth. We were provided with three days' notice. A
week later, asteroid 1996 JG came within 2,000,000 miles of hitting
Earth. We did not even see it until it had already passed. Both were
twice as large as the Tunguska asteroid, and either could have killed
one percent of Earth's human population; that is, 60 million people.

With the acceptance of the impact hypothesis has come another, related
area of study: the frighteningly prominent role smaller impacts are
now thought by some to have played in known human history. Asteroids
themselves have traditionally been subject to great, if puzzled,
veneration. In ancient China, for instance, people used to grind up
and eat meteorites, and other cultures took the metal from M-type
meteorites and smithed them into weapons. Simply put, it is very
likely that smaller catastrophic impacts have occurred far more often
than most people realize. A l,000-megaton event in Argentina resulted
in mile-wide craters not thought to be more than about 10,000 years
old. A small (not quite mass) extinction occurred 10,000 years ago,
which finished off, primarily in the Americas, larger terrestrial
mammals such as the woolly mammoth and the saber-toothed tiger. If we
project forward another 6,000 years, we watch several fairly advanced
civilizations, such as that of Ur, fall and vanish. We find that
bookkeeping of the skies becomes, quite suddenly, a matter of some
cultural importance. The literature of the ancient cultures that
proceeded from Ur, including that which became the Judeo-Christian,
contains much lore of a vast flood, lost continents, vanished oceans,
a world gone meteorologically mad. We find Egypt, which traced its
origins to Pythom, apparently a falling object of some kind, and we
find a strange and seemingly offhanded Egyptian memory, now forever
enshrined in Exodus, of a darkness lasting three days. We find an
ancient city in Arabia, Wabar, supposedly destroyed by another falling
object, an event that happens to coincide with the rise of Babylonian
astrology. There are counter explanations to all of these
developments, of course, just as heaven is a counterargument to
death-it exists primarily because we need it to. If it is true that
small though still catastrophic impacts-caused by asteroids not big
enough to find with existing technology -are statistically timed to
occur roughly every few thousand years, this means that the woolly
mammoth and Ur fell victim to separate impacts, and it means that
another is due, well, any day now.

Unlike ancient human beings, however, we actually can do something
about this other than devise cosmologies of desperate emotional
necessity. There are several theoretical defenses to asteroid impacts.
One method involves landing a rocket-booster device on the
collision-course asteroid and then attempting to steer it from Earth's
path. Another proposes placing solar panels on an incoming asteroid to
create what is called a "solar-powered mass driver," thereby altering
its trajectory in a fuelless, less-expensive way. Yet another
envisions painting asteroids black in the hopes that the change in
absorbed solar radiation would gradually shift the object's orbit. But
the most imperative theories involving asteroid defense are nuclear.
It seems beautifully, if demonically, apt that the weapons that have
terrified us all for half a century might turn out to possess the
cleansing holy fire we were all promised they did. How effective they
would be against the unideological phenomenon of asteroids is another
matter. Simply hitting the rocks with missiles would accomplish
nothing; they are traveling too fast, and the warheads' tonnage would
only be absorbed. The most useful deployment of nuclear force against
an unopposable asteroidal object would be to explode a powerful nuke
at the asteroid's surface near the closest point its orbit takes to
the sun, where the leverage of deflection is greatest. Then our planet
would wait. If this first explosion did not alter the asteroid's
course enough (chances are it would not), one would try another
explosion, and perhaps another, thus "herding" the asteroid into a
different orbit. Problems with this method, and there are many,
include the possibility that the asteroid in question has been
weakened by an older collision, or is loosely bound together (a rubble
pile, this is called), or has an insecure interior riddled with open
space. Nuclear weapons, used against such unstable bodies, might only
transform one asteroid coming our way into several asteroids, perhaps
hundreds-pieces of which would be large enough to breach Earth's
atmosphere. The other problem would be an enlivened nuclear-weapons
industry, one busily developing warheads not as a deterrent but as
devices intended to be used. No one would object, of course, if we
were to keep a few of these newer-line missiles-which would, no doubt,
be better than ever-on high alert, or deployed a few more here or
there. There are maniacs out there. Maniacs with nuclear weapons!
China, for its part, has explained its resistance to the
anti-nuclear-proliferation Comprehensive Test Ban Treaty on the
grounds that it would like to keep its missiles in the event of an
asteroid-impact threat. No doubt Iran and Iraq and Libya and Algeria
would like to step up their own programs, too. Just in case.

HELPLESS

Reading about asteroid impacts will undoubtedly cause many people
distress. I feel bad about that, and I would like to say that although
these threats are terrifying all is not lost. Concerned, dedicated
people are working on the asteroid-impact threat, and one need not be
a deluded idealist to believe that they may succeed. Hope, after all,
takes as its foundation not likelihood but possibility. There is,
however, another threat to ourselves and our civilization, one that
cannot be stopped or avoided. You readers who find yourselves already
traumatized, let me entreat you here, please, to stop reading.

Comets differ from asteroids in several ways. Consensus holds that
they are "dirty snowballs" made up of ice and carbon-bearing rock. A
1986 "flyby" mission to Halley's comet beamed back data suggesting
that its nucleus is less dense than water, 50 percent of it a warren
of cracked and empty networks. How this pertains to other comets is
not known. Before the comet Hyakutake was found in 1996, only five
previous comets had been detected, by radar. Traveling like frozen
freight trains along the loneliest edges of the solar system, comets
occasionally enter the inner solar system, the neighborhood of Earth,
at twenty-six miles a second, leaving behind them a long tail of dust
and gas crystals that can stretch back as far as sixty million miles.
Replacement dust accumulates on cometary surfaces; when the dust layer
becomes thick enough, comets gain an excellent shield against solar
heating. An icy skein builds, and they get bigger. One cometlike
object, Quaoar, recently found floating out around Pluto, is 800 miles
across. As comets approach the sun, however, their frozen gases expand
and form makeshift jets that can alter their course. This makes
predicting accurate orbits of newly discovered comets nearly
impossible. But discovering them is also challenging. They are too
fast, not typically seen until they pass near the sun, and in any
event their gas- and dust-obscured passage across the universe's dark
starry backfield is often difficult to discern.

We know of two types of comets. The closest to Earth, called
short-period comets, are found just beyond Neptune in the Kuiper Belt,
named after the astronomer Gerard Kuiper. Long-period comets make up
what is known as the Oort cloud-named in honor of the astronomer Jan
Oort, who first hypothesized its existence-an envelope of as many as a
trillion comets that travel around the sun far beyond Pluto. The
Kuiper Belt and Oort cloud are both remnants of the early physical
conditions of the primitive solar nebula. The Oort cloud was most
likely formed by gravitational ejecta from the Uranus and Neptune
regions, and some astronomers believe that the Kuiper Belt is merely
the innermost edge of the Oort cloud. Jupiter's massive gravitational
force shields Earth from many long-period comets, but its movement is
also thought to be the key mechanism for injecting the few comets that
manage to get past it, often in shattered form, into short-period
Earth-crossing orbits. Almost all long-period comets have orbital
periods of 100,000 years or more, making it all but certain that there
are literally millions of comets with periodic near-Earth passes we
know nothing about. We could have as little as three months' 'warning
when a comet on a collision course with Earth appears in the sky, Most
are too big to stop with nuclear weapons, which does not much matter,
as their meddled-with, chaotic trajectories make intercepting them
fantasy.

Edmund Halley, in his A Synopsis of the Astronomy of Comets (1705),
was the first scientist to wonder if comets might impact Earth.
Halley's astonishing foresight was only negligibly explored until the
1970s. In the 199Os, however, comets stepped to the forefront of
scientific thinking about apocalyptic mass extinction. On March 24,
1993, three years after concluding that the "Earth resides in an
asteroid swarm," Eugene Shoemaker, along with his wife, Carolyn, and
the amateur (though highly respected) astronomer David Levy, detected
upon a photographic plate a strange smear of pearly light. What they
had found was twenty-one pieces of a comet that had been torn apart by
Jupiter's orbit the previous July. The pieces of this comet, now
called Shoemaker-Levy 9, were predicted to impact the planet's surface
the following year. Sadly, the impact would occur on Jupiter's dark
side, viewable only at a great distance by the Voyager and Galileo
probes. Many worried that humankind's first opportunity to observe a
large-body impact in at least 800 years (or since what may have been
the formation of the moon's Giordano Bruno crater, in 1178) would be
spoiled.

They needn't have worried. Upon impact, the larger pieces of
Shoemaker-Levy blew fireballs thousands of kilometers high into
Jupiter's atmosphere and were plainly visible through telescopes on
Earth. When Fragment G collided with the King of Planets two days
after the first impact, the flash was so bright that infrared scopes
all over Earth were momentarily fried. The scar left by Fragment G was
larger than Earth itself, and the explosive energy released was the
equivalent of a Hiroshima-sized nuclear bomb exploding every second
for thirteen years. Shoemaker-Levy's volatile swan song was quickly
and accurately called the astronomical event of the century, and has
proved the starkest challenge so far to humanity's sense of its own
inviolability.

Scientists are divided on whether the K-T mass extinction sixty-five
million years ago was caused by a comet or an asteroid. The severity
of the event-miles of evaporated ocean; the very high chance that a
hundred trillion tons of molten rock were thrown into space, frozen,
and then pulled back down to the surface of Earth in the form of more
impacting meteorites; an ozone layer so shredded that any creature
peeking out of its cave even a year after the impact would have found
its skin on fire in the ultraviolet spring; the sheer number of
extinctions-points to a comet. It is empirically inarguable that every
few dozen million years a mass extinction is visited upon Earth.
Various arguments place these mass extinctions, the extent and
agencies of which are still debated, at intervals ranging from
twenty-six to thirty million years. A theory called the "Shiva
Hypothesis," named after the Hindu god of destruction, holds that mass
extinctions occur in startling simultaneity with the movement of our
solar system through the galactic plane, a passage that is thought to
perturb millions of Oort cloud comets into our path. Comets, and the
mass extinctions they cause, might very well be the piston that drives
Earth's biological processes. If the Shiva Hypothesis is correct, we
are all just marking time until the next comet arrives.

Before leaving the Jet Propulsion Laboratory, I stopped to visit Don
Yeomans, a man commonly regarded as one of the world's key figures in
near-Earth object studies, to talk about comets. Yeomans struck me as
a former nerd who has aged extremely well. He radiated a thoughtful,
deceptively low-key intelligence. That he is well liked was clear from
the boxed Yeomans action figure someone had given him, and from the
fact that the writers of a doomsday television movie named their
impacting comet after him. When he went through his files to find the
film's title, I noticed that one of his drawers was labeled "NEOs and
Things That Go Bump in the Night . . ."

"We're hit by tons of material every day," Yeomans told me after his
futile file search, "but it's all dust. We're all walking around with
comet dust in our hair. What really interests me is that comets and
their impacts with Earth are - apart from the sun - the only bodies
that have a direct effect on evolution, on life, on the origin of
life. They probably brought the materials of life to Earth -
carbon-based molecules and water. They're not just something in the
sky that looks nice, like the outer planets or stars or clusters.
That's all very interesting but has no real effect on us. When you
come right down to it, the guy in the street wants to know, 'What's in
it for me? Why should we be studying these things?' And comets, I
always claim, pass the brother-in-law test."

"The brother-in-law test?"

Yeomans smiled. "My brother-in-law doesn't believe in the space
program that much. And he says, 'Hey, why are you guys spending
millions of dollars to do this and that?' But after I explained comets
and the origin of life and evolution, he said, 'Maybe that makes
sense."'

Yeomans was addressing the notion that impacts upon the ancient
atmosphere, before the origin of life on Earth, generated a stew of
simple organic molecules that eventually resulted in amino acids and
nitrogen bases able to serve as the keystones of life. Impacts are
high-energy processes not unlike ultraviolet light, cosmic-ray
irradiation, and lightning discharges. The atmospheric violence caused
by impacts can also create a high incidence of these potentially
life-creating processes-a case of God appearing in the whirlwind,
hurricane, earthquake, and tsunami.

Human beings are not a goal that this stew evolutionarily pushed
toward; they are only one entity within evolution. Contrary to the
worries of certain Kansas school boards, evolution does not, in fact,
push toward anything. The Darwinian evolution attacked by creationists
is not even Darwinian but an inflexible version of natural selection
promoted by Darwin's followers that Darwin himself would not have
recognized. Today, Darwinian natural selection is an acknowledged fact
of life's micro development, but on a macro scale natural selection is
moot. How does an organism adapt when its world is hit without warning
by a ten-mile-wide ball of ice traveling 70,000 miles per hour?
Evolution is not even good for the planet, based as it is on
speciation and phyletic transformation. If these processes were
allowed to go on indefinitely the planet would be overrun and all life
would go extinct. Thus mass extinction serves as speciation's
necessary foil, and the victims run well into the millions: today's
extant species account for less than one percent of the total number
to have ever lived. "Mass extinctions," wrote Stephen Jay Gould, "can
derail, undo, and reorient whatever might be accumulating during the
'normal' times" between "imparting a distinctive, and perhaps
controlling, signature to diversity and disparity in the history of
life."

"These objects are weird," Yeomans concluded. "And the history of
these objects is weirder still. Comets are unlike anything else. They
show up unexpectedly, they disappear unexpectedly, they have different
shapes and sizes and characteristics. There's nothing
celestial-looking about them. They are the wild cards."

By the time I left Yeomans's office, The End of the World and its
wild-card causes had begun to seem less like a force of impartial
terror than something far more complicated-something necessary . Life
is a huge blackboard tilled with a million marks of chalk. Every
thirty million years that chalkboard is forcefully wiped clean,
leaving only a few small smudges in the comers, whereupon life begins
again without regard to perfection of adaptation, what has come before
it, or the miserable consciousnesses of those few creatures able to
wonder why they are here. For reasons no one yet understands, smaller
animals seem to have a survivalist edge in the aftermath of most mass
extinctions. One such (most likely shitlessly frightened) animal, a
primate, survived the K-T mass extinction. Possibly it lived in a wet,
boggy area, the only part of the planet that would not have burned up
in the global firestorm. Say its name: Puratorius. We may have this
tiny creature to thank for our current civilization. So impacting
comets giveth, clearing the way for new species, and they taketh away.
This most unstoppably terrifying vision of the world's end is also the
most comforting, for it forms an iron law no organism, including ours,
can hope to step around. Life cannot win. "Behold," the Lord says in
Isaiah 65:17, "I create new heavens and a new earth: and the former
shall not be remembered, nor come into mind." There is freedom in that
recognition, a sense of knowing where we stand in fighting to stave
off oblivion, and of knowing where we surrender. As everything must.

**Tom Bissell's last article for
Harper's Magazine, "Eternal Winter, " appeared in the April 2002
issue. His first book, an expansion of that piece, will be published
in September by Pantheon.

*Harper's
Magazine,
February 2003 page 33-47.

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